With a Pt-SiO2 catalyst n-octane underwent dehydrocyclization in the presence of D-2 or with an equimolar mixture of n-octabe-d(0) and n-octane-d(18) without a measurable extent of HID exchange in the unconverted n-octane. When similar experiments were conducted with a Pt-Sn-SiO2 H/D exchange occurred. The results suggest that tin functions to decrease the strength of chemisorption of n-octane at 482 degrees C so that adsorption is reversible in contrast to the irreversible adsorption for a Pt-SiO2 catalyst, The presence of tin allows HID exchange to occur in the unconverted n-octane.